The present invention relates to a complex lens that consists of a plurality of stacked lens portions, and particularly, relates to the complex lens for a tandem scanning optical system employed in an imaging device such as a color laser printer for converging a plurality of light beams deflected by a deflector. Further, the present invention relates to a manufacturing method of such a complex lens for a tandem scanning optical system.
A tandem scanning optical system employed in a color laser printer is provided with four semiconductor lasers and four photoconductive drums that correspond to colors Y (Yellow), M (Magenta), C (Cyan) and K (blacK), respectively. In such a tandem scanning optical system, it is preferable to make at least one part of the optical system shareable among the colors to downsize the system. The polygon mirror may be shared.
When a polygon mirror is shared, four light beams are incident on the polygon mirror such that they are arranged in an auxiliary scanning direction, which is coincident with a direction of the rotation axis of the polygon mirror. The four light beams deflected by the polygon mirror are converged by an fθ lens and the optical paths thereof are separated by mirrors. The separated four light beams form scanning lines on the respective photoconductive drums.
It is preferable that the four light beams deflected by the polygon mirror are converged by the respective lens elements in order to obtain the most suitable optical performance. On the other hand, the smaller the thickness of the polygon mirror is, the smaller the spaces among the four light beams are in the vicinity of the polygon mirror. This does not allow employing independent lens elements for the respective light beams. Therefore, a lens in the fθ e lens arranged close to the polygon mirror should be a complex lens that consists of stacked four lens portions.